The sheer weight of the rope in the hoisting system of a conventional elevator limits their practical length of travel. To reach portions of tall buildings which exceed that limitation, it has been common to deliver passengers to sky lobbies, where the passengers walk on foot to other elevators which will take them higher in the building. However, the milling around of passengers is typically disorderly, and disrupts the steady flow of passengers upwardly or downwardly in the building.
All of the passengers for upper floors of a building must travel upwardly through the lower floors of the building. Therefore, as buildings become higher, more and more passengers must travel through the lower floors, requiring that more and more of the building be devoted to elevator hoistways (referred to as the "core" herein). Reduction of the amount of core required to move adequate passengers to the upper reaches of a building requires increases in the effective usage of each elevator hoistway. For instance, the known double deck car doubled the number of passengers which could be moved during peak traffic, thereby reducing the number of required hoistways by nearly half. Suggestions for having multiple cabs moving in hoistways have included double slung systems in which a higher cab moves twice the distance of a lower cab due to a roping ratio, and elevators powered by linear induction motors (LIMs) on the sidewalls of the hoistways, thereby eliminating the need for roping. However, the double slung systems are useless for shuttling passengers to sky lobbies in very tall buildings, and the LIMs are not yet practical, principally because, without a counterweight, motor components and energy consumption are prohibitively large.
In order to reach longer distances, an elevator cab may be moved in a first car frame in a first hoistway, from the ground floor up to a transfer floor, moved horizontally into a second elevator car frame in a second hoistway, and moved therein upwardly in the building, and so forth. Since the loading and unloading of passengers takes considerable time, in contrast with high speed express runs of elevators, another way to increase hoistway utilization, thereby decreasing core requirements, includes moving the elevator cab out of the hoistway for unloading and loading.
In buildings which are sufficiently tall to have banks of shuttle elevators that are many hundreds of meters high, particularly when such buildings have 24 hour usage (such as residences) it is unlikely that adequate emergency service, such as fire and medical, can be provided utilizing equipment dispatched from the ground.